Graphene coupled MIM rectifier especially for use in monolithic broadband infrared energy collector

ABSTRACT

A rectifier comprising a metal-insulator-metal (MIM) structure. The insulator may be a native oxide with an adjacent layer of graphene. In one implementation, the rectifier is used in an electromagnetic energy collector consisting of a planar waveguide formed of multiple material layers having at least two different dielectric constants. MIM rectifiers are aligned with mirrors are formed within the waveguide core. In some arrangements, a plurality of MIM rectifiers are disposed in a column or 3D array beneath each mirror.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of a U.S. Provisional Patent Application Ser. No. 62/040,509 filed Aug. 22, 2014 entitled “GRAPHENE COUPLED MIM RECTIFIER FOR USE IN INFRARED ANTENNA”, and is also a continuation-in-part of copending U.S. patent application Ser. No. 14/533,614 filed on Nov. 5, 2014 entitled MONOLITHIC BROADBAND ENERGY COLLECTOR WITH TIR MIRROR AND DETECTOR ARRAY which claims the benefit of a U.S. Provisional Patent Application Ser. No. 61/962,396 filed Nov. 6, 2013 entitled “MONOLITHICALLY INTEGRATED PLANAR BROADBAND WAVEGUIDE AND SOLAR PHOTODETECTOR”. This application is also a continuation-in-part of copending U.S. patent application Ser. No. 14/078,755, entitled “WIDEBAND LIGHT ENERGY WAVEGUIDE AND DETECTOR” filed Nov. 13, 2013. The entire contents of each of the above-referenced patent applications are hereby incorporated by reference.

BACKGROUND

Technical Field

This patent application relates to an electromagnetic energy collection device and specifically to rectifiers for energy harvesting or general rectification of ultra-high frequency energy.

Background Information

The integration of photonic devices for low-cost on-chip optical interconnects is of great interest for high speed computing. Additionally, there is a desire in the solar energy industry to tap the potential high theoretical conversion efficiency of rectennas, where highly concentrated coherent light in a waveguide could enable higher rectenna efficiencies.

One of the key elements for both end uses is the efficient integration of a waveguide and a detector. Coupling of light from a planar waveguide to a monolithically integrated detector, such as a tunnel diode or rectenna, can be accomplished in several ways, but it has remained a challenge to do so with high efficiency across a broad spectral band.

There are a variety of applications for tunnel diodes and rectennas, including energy harvesting, rectification, and harmonic mixing for down and up conversion. Dipole, bowtie and spiral have been some of the antenna geometries explored, and the rectifying diodes include p-n junction semiconductor diodes, Schottky metal-semiconductor diodes, and MIM tunnel diodes. The choice of rectifier is dependent on the frequency of electromagnetic energy. The diodes listed above have increasing bandwidth as they rely less upon a capacitive depletion layer for asymmetry of current flow. For conversion of terahertz (THz) frequencies, tunnel diodes made from metal-insulator-metals are the only type of junction to that has the speed to rectify IR and potentially visible light.

Although there has been some success in fabricating dissimilar MIM diodes and obtaining rectification of infrared (IR) energy, the efficiencies reported have been less than 1% due to the limited asymmetry in the current vs. voltage characteristics of the diode.

Some earlier work experimented with creating a Schottky-like diode from a single layer of graphene in contact with a metal electrode, as reported by Nourbakhsh, A, et. al, Applied Physics Letters, 97, 1631010 (2010).

Reference may also be made to rectenna apparatus for solar energy collection, such as in U.S. Pat. No. 8,437,082 entitled “Orthogonal scattering features for solar array” and U.S. Pat. No. 8,735,719 entitled “Leaky solar array with spatially separated collectors”, and U.S. Patent Publication 2014/0182656 entitled “Wideband light energy waveguide and detector”, each assigned to AMI R&D, the assignee of the present application, and each of which are hereby incorporated by reference.

SUMMARY

A rectifier is made from a metal-insulator-metal (MIM) structure, whereby the insulator consists of a native oxide and a doped or undoped layer of graphene. By adding a layer of graphene, we further optimize the work function stack between the metals, and create a new metal-insulator-graphene-metal tunnel junction for the rectification of high frequency electromagnetic energy.

The rectifier is of particular use in a monolithic planar light or other electromagnetic energy concentrator module. Diode rectifier detectors (in the case of a solar application) or antenna structures (more generally) may be disposed adjacent a waveguide core. An optional multi-layer cladding restricts angular deviation; certain optional embodiments may include gratings, mirrors, or spectrally filtered couplings.

In one implementation, an electromagnetic energy concentrator includes a prism, and a waveguide aligned with the prism. The waveguide includes a waveguide core layer. A gap layer of uniform thickness may be disposed between the prism and waveguide. Energy detectors, which may be graphene-coupled rectifiers described herein, are disposed adjacent to and co-extensive with a second surface of the waveguide. The detectors may be placed in one or more cladding layers on the second surface of the waveguide.

In one arrangement, the detectors (MIM rectifiers) are operative in each of at least two bands, and the horizontal spacing between detectors operating in a given band depends on the wavelength of that given band. The detectors may also be vertically positioned with respect to the prism, such that that distance depends on 1/e.

The detectors may be disposed in a homogenous carrier layer, or may be disposed in two or more layers such that detectors operative in one band are disposed in a different layer than the detectors operative in another band.

In still other implementations, one or more internal reflection mirrors are embedded in waveguide to efficiently direct solar energy to the detectors.

In still other implementations, the detectors may be nantennas disposed in vertical series adjacent and below each mirror. These may also include a plurality of bowtie shaped antenna elements arranged as an array in at least two dimensions.

Other arrangements are possible, as described below and as particularly set forth in the claims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention.

FIG. 1 is a prism coupled optical waveguide using spatially separated photovoltaic collectors;

FIGS. 2A and 2B illustrate a homogeneous and multi-layer arrangement, respectively;

FIG. 2C is a multi-layer gap layer;

FIGS. 3A and 3B show gratings disposed adjacent the homogeneous and multi-layer configurations;

FIG. 3C shows one possible homogeneous implementation in more detail;

FIGS. 4A and 4B illustrate embedded material strips and the resulting improved efficiencies possible;

FIGS. 5A and 5B show a multi-layer photovoltaic arrangement in more detail with an index profile;

FIGS. 6A, 6B, 6C, and 6D show effective indexes for various configurations;

FIG. 7 is an implementation of a solar collector using a dichroic filter stack;

FIGS. 8A and 8B show a broadband coupler using an array of embedded Total Internal Reflector (TIR) mirrors with either continuous or discrete detectors;

FIG. 9 illustrates intrawaveguide spectral splitting;

FIG. 10 shows the intrawaveguide spectral splitting in more detail as simulated;

FIG. 11A is a cross sectional view of a MIM type rectifier used as a detector;

FIG. 11B is an embodiment using multiple MIM detectors in a vertical stack;

FIG. 12 is a result of simulating two stacked antennas;

FIGS. 13A and 13B show a stacked bowtie array and the resulting power field;

FIG. 14 shows a bowtie element in more detail; and

FIG. 15 is a plot of receive efficiency for different bowtie element sizes.

DETAILED DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT

1. Introduction

The metal-insulation-metal (MIM) type rectifier described herein is particularly useful as a detector in an energy collection apparatus that uses a planar waveguide. There are many applications for coupling energy into a thin optical waveguide. Most techniques for coupling, such as grating coupling and prism coupling, are inherently narrowband due to the dispersion of the waveguide. In the case of prism coupling, the incoming wave typically must be a plane wave, and must also be input at an angle that is commensurate with the effective index of the waveguide, given the prism's inherent dispersion over the spectral band of interest.

FIG. 1 is one example of a prism-coupled waveguide approach to a solar collector that improves over other designs. A prism 100 is placed over a waveguide core 102; the waveguide core 102 may also be composed of one or more cladding layers 103 placed on a face of the core 102 opposite the prism 100. The waveguide core 102 has a major axis (the horizontal plane in FIG. 1) aligned with the prism 100. A gap layer 101 is preferably disposed between the prism 100 and the waveguide core 102. One or more detectors such as (MIM type structure described below) 105 are disposed within or adjacent (that is, below) the cladding layers 103.

Incoming light reaches the prism 100 and is then coupled to the waveguide core 102 after travelling through the gap 101. The incoming light is coupled at angle θin to a prism face of angle θp to the horizontal plane. This angle θin of coupling is defined in equation (1) (below) as a function of the index of refraction of materials (np for the prism, and neff for the effective index of the waveguide.

$\begin{matrix} {\theta_{in} = {\arcsin\left\{ {{np}*{\sin\left\lbrack {\theta_{p} - {\arcsin\left( \frac{neff}{np} \right)}} \right\rbrack}} \right\}}} & (1) \end{matrix}$ where

-   -   np refers to the prism 100 index of refraction;     -   ng refers to the gap 101 index of refraction;     -   ncore refers to the core 102 index of refraction;     -   nclad refers to the cladding 103 index of refraction; and     -   neff is the effective index of refraction of the core and         cladding together.

By optimizing the material used for the cladding 103 and geometric properties to control the desired effective index of the waveguide (neff), the collector can be optimized to produce a single or narrow range of angles that will allow a large spectral bandwidth to be efficiently coupled into the waveguide 102.

Previous attempts at altering the prism 101 material properties for a given waveguide have, via modelling, produced a 110 nm spectral bandwidth in the visible wavelength region. There a prescription for a fictitious material was designed; see Optics Communications 136(1997) 320-326, Mendes, Sergio B., et al., “Achromatic prism-coupler for planar waveguide”. With the structure in FIG. 1 we have obtained less than a 0.4 degree beam width from 400 nm to 1200 nm, or an 800 nm spectral bandwidth, using actual materials that have already been produced with known chemical vapor deposition recipes.

As mentioned above, one or more dielectric cladding layers 103 is desired to abut the waveguide core 102. However, this can present a problem during integration and assembly of the waveguide 102 and detectors (also called “cells” herein) 105. For better efficiency, the cells 105 should abut the waveguide core 102, and so they cannot be bonded to the cladding 103. In one embodiment therefore, there is no physical cladding layer 103 beneath the waveguide core; rather air is used as the dielectric “cladding”. This simplifies the manufacturing process by allowing the waveguide core 102 to be bonded directly to one or more cells 105. In this configuration, due to the lack of any physical cladding material, the air beneath the waveguide core is considered the cladding of the waveguide. This type of waveguide actually confines fields within the core to a greater degree than other arrangements.

This “air cladding” arrangement also lends itself to an improved manufacturing process. Using the bottom of prism 100 as a substrate, coupling layer 101 is first deposited. Next, the core waveguide 102 is deposited, and then a third layer consisting of periodically spaced cells 105 can be deposited or affixed to the core waveguide layer. An electrical backplane (not shown) can then be affixed to the assembly.

2. Horizontal Position of Detectors Depends on Wavelength

Continuing to refer to FIG. 1, a plurality of detectors (cells) 105 of different wavelengths appropriate for each band (that is, sets of cells 105 corresponding with each wavelength region of the solar band of interest) line the waveguide 102 and/or cladding 103 surfaces where collected energy is incident.

In a preferred arrangement, there are multiple cells 105 disposed along the waveguide surface at one or more collector regions 106-1, 106-2, . . . , 106-5. For each distinct solar wavelength band of interest, there exist cells that are very efficient. Tuned dielectric coatings (not shown in FIG. 1) can be added to or near each cell 105 to accept only in-band solar energy of interest.

The solar wavelength bands can be divided in any number of ways, including 1) multiple single-junction, single-band detector devices, 2) multiple multi-junction, multi-band devices or 3) any combination thereof. The actual division and distribution of these options is optimized based on the responsivity of each device, with respect to overall efficiency across the entire solar band.

In the structure shown in FIG. 1, a long, constant thickness coupling layer 101 formed of a material such as MgF and of the order of several millimeters is desirable for several reasons. The long coupling layer lends itself to the use of a large prism wedge 100 covering a multitude of focusing regions which would ordinarily require a wedge for every focusing region. This allows a relatively long layer 103 to be used with the single prism 100. Another advantage of providing a correspondingly long layer 103 is the ability to increase the concentration of energy in the longer wavelength bands, since the efficiency of a photovoltaic device increases with concentration. In previous configurations, a focusing region of 50 microns and an exponential taper in the coupling layer were used to keep the beam width at 0.5 degrees and maintain efficiency at 400 nm. The disadvantage of this 50 micron constraint is not only loss of concentration at the longer wavelengths, but the difficulty of manufacturing an exponential taper.

As shown in FIG. 1, using a relatively long, constant thickness coupling layer 101 allows multiple arrays of detectors 105 to be used in each collector. As shown in one example embodiment of FIG. 2A, each of the three solar bands will have its own detector array, optimized for each of a respective one of a high band gap 105-h, medium band gap 105-m, and low band gap 105-1. In FIG. 2A the three bands are shown separately with the positions of the detector devices along the separate layers. The incident sunlight excites the fields in the layers as shown by the dotted lines. The fields are seen to increase in the focusing regions until most of the field is absorbed by the detector devices 105. This behavior is repeated along the layer with complete absorption of the incident field resulting.

In the illustrated implementation the spacing between the 400 nm photovoltaic detectors is 50 microns. The detectors operating at longer wavelengths are disposed with progressively wider spacing to coherently absorb more light while maintaining a 0.5 degree beamwidth. For example, the spacing of the 800 nm detectors can be 100 microns, and that of the 1200 nm detectors 150 microns. Thus, the spacing between detectors in layer(s) 103 depends on the respective operating bands of the detectors as well as the desired beamwidth.

In another arrangement, a single composite layer 103-c is seen in FIG. 2B that contains all of the detectors 103-1, 103-m, 103-h.

There is possibly a slight wavelength dependency in the coupling layer 101 if the operating wavelength range is from 400 nm to 1200 nm. That dependency, if considered critical, can be mitigated by implementing a three layer coupling layer 101 in some embodiments, using the configuration shown in FIG. 2C. Magnesium fluoride is the 1.4 dielectric constant material while titanium dioxide is the 2.5 material.

The aforementioned waveguide design was intended for the capture and concentration of solar energy, although the design can be tailored for operating in other optical wavelength regions, such as those used for optical communication systems.

3. Vertical Position of Detectors Depends on Wavelength

Evanescent coupling of light within the waveguide to cells 105 can be further encouraged by matching the detector cells' 105 penetration (e.g., vertical position) in the waveguide 102 to their respective 1/e field strength confinement location. This location is defined as the depth (or penetration) into the waveguide at which the cross-sectional power is 1/e of its peak value. The power density for the lower wavelengths is greater at the core 102 center; this is because as the wavelength increases, more energy travels down into the cladding layers 103. The result is that the light activating the relatively high energy band gap cells 105-h (those which are operative at the shorter wavelengths) will penetrate the furthest into the waveguide, and the light activating the lower band gap cells 105-1 (operative at the longer wavelengths) will penetrate the least, as shown in FIG. 1.

As higher band gap cells 105-h typically have a higher absorption coefficient, the coupling length is shorter, and these cells will be smaller in width along the horizontal. If one considers the cells 105 as elements in an antenna array, it will be seen that the separation between the detectors will be commensurate with their wavelength band or band gap. This is also depicted in FIGS. 1, 2A and 2B. As such, the spectral beam width will be equalized across the solar spectrum.

Selectively choosing the relative depth position of the detector 105 within the waveguide thus acts as a spectral filter for coupling the desired portion of the spectrum to the respective detector cell 105. This allows for maximum efficiency of the spatially separated band gap optimized cell architecture. It also potentially alleviates the need to place band pass filters on the cells 105.

FIGS. 1, 2A and 3A show this preferred vertical placement of the different band gap cells (as suggested above). The shorter wavelength (high band gap) cell 105-h is encountered first, and since the shorter wavelength energy is less confined, a portion of the energy transits into that cell via leaky-wave propagation (the detector is a higher index than the waveguide). The middle and longer wavelength energy is unaffected because it is more confined and travels vertically down the waveguide until it encounters the next detector, optimized for middle wavelength energy. The middle wavelength energy leak-wave propagates into the middle wavelength band-gap optimized detector 105-m. This process continues until longer wavelength (low bandgap energy) detector 105-1 is encountered.

The distance between the detector(s) 105 and the waveguide core 102 is optimized such that the detectors can absorb the maximum amount of energy per unit length from the waveguide 102, without impeding or affecting the adjacent band.

4. Homogeneous and Multi-Layered Detectors Layers

FIGS. 3A and 3B show the implementations for a multi-layer detector and single (homogeneous) detector layer embodiment in more detail.

Shown specifically is a region such as region 106-3 containing all three band gap detectors. In FIG. 3A the shortest wavelength (highest band gap) detector 105-h active from 400-580 nm, is located in a layer 103-h closest to waveguide 102. The longest wavelength (lowest band gap) detector 105-1 (active from 840-1200 nm) is located in a layer 103-1 farthest away from waveguide 102. The medium band gap detectors 105-m (active from 580-840 nm) are placed in a middle layer 103-m.

5. Gratings Placed Near Detectors

FIGS. 3A and 3B also show energy from 400 to 1200 nm propagating from left to right and interacting with various grating structures 210 placed adjacent each detector 105. The grating structures 210 further assist with directing light energy of the appropriate wavelength into the corresponding detector.

The propagation to each respective detector 105 is governed by the element spacing of each grating 210. For a given element spacing, the longer the wavelength and the less propagation, until finally a cutoff wavelength is reached where propagation is zero. For example, the element spacing for grating 210-h is chosen such that is can direct all of the light energy at 400-580 nm into its corresponding 400-580 detector 105-h, while light at the longer wavelengths is not. The same can be said of the grating 210-m, which delivers all energy it receives at 580-840 nm into its corresponding detector 105-m, with the 840-1200 nm band energy passing through unattenuated until it reaches grating 210-1.

Towards this end, the spacing of the elements that make up each grating preferably changes along the length of each grating 210, in a chirp like fashion. Comparing the various gratings 210-h, 210-m, 210-1, it is also preferred that they collectively exhibit a chirp taper to their element lengths.

The size of the elements that make up each grating 210 can also similarly vary. For example, the grating element size used for grating 210-h used with the 400 nm detector 105-h can be ⅓ the size of the grating element 210-1 used for the 1200 nm detector 105-1. Tapering the element size in this way acts as a high pass filter between the waveguide 102 and the photovoltaics 105, since Rayleigh scattering is involved where the element response is 1/(wavelength) to the fourth power.

The chirp spacing between elements of each grating can be determined by λ/2 where λ is the wavelength at the center of the band of interest of each respective photovoltaic, and where there are approximately 10 grating elements per photovoltaic.

The size of the larger elements can also be made relative to the element size chosen for the shorter wavelengths (e.g., 400-500 nm). See FIG. 3C for an example. The overall length of each grating may, for example be 13 microns.

6. Embedded Material Strips Equalize Dispersion in Prism and Waveguide

For the traveling wave structure to receive solar energy over a broad band, high efficiency in receive must be achieved without changing the elevation angle. To do this, one should achieve a uniform mode match for both horizontal and vertical polarizations across frequency, between the wedge and the waveguide. Because most materials suitable for use as the prism 100 are inherently dispersive across the solar band, we correct for dispersion within the waveguide to “mode match” across the entire band. This mode match involves matching dispersion curves of the prism 100 and waveguide 102. When this is achieved, the concentrator has high receive efficiency at the same receive angle across all wavelengths of interest. The concentrator is then aimed at the source (the sun) at this angle.

This mode matching can be achieved over a broadband and over polarization by inserting strips of material 230, preferably rectangular in shape, within the waveguide 102 that have the same dispersion characteristics of the prism 100. FIG. 4A is an example implementation. The height to width ratio of these strips 230 is experimentally determined until mode matching occurs over a broad band. The width (w) and height (h) are controlled independently, and each affect orthogonal polarizations independently.

The illustrated example involved a TiO2 (titanium dioxide) prism 100 and a core 102 waveguide formed of SiO2 (silicon dioxide). The correcting strips 230 (made of TiO2) are embedded into the SiO2 and optimized to obtain the desired performance. FIG. 4B is one such set of performance curves.

The performance can be characterized by the equation

${\cos(\theta)} = \frac{\beta({waveguide})}{\beta({wedge})}$ where the β's are the respective propagation constants of the waveguide 102 and the wedge 100. The sizes of the strips 230 are adjusted until the ratio is constant over all wavelengths of interest and both the horizontal and vertical polarizations.

When the desired mode matching is achieved, receive efficiency lines up within the same angular region, as depicted in the receive efficiency chart in FIG. 4B. This figure was generated using a High Frequency Structural Simulator (HFSS) model that included both the horizontal and vertical polarization across the 400 nm to 1200 nm range.

7. Waveguide Composed of a Core and Multiple Cladding Layers

The device can also achieve dispersion tailoring using a multi-layered cladding 103, applicable to both the solar case and operation at other wavelength regions. The particular region chosen for proof of design was 400 nm to 1200 nm.

Given a prism 100 made from Ohara S-LAH65 with a 55 degree angle (θp), the waveguide 102 design is expected to produce a single angle of incidence coupling over a 400 nm to 1200 nm range by using five (5) closely spaced refractive index layers 103 of specifically designed thickness to tailor the waveguide effective index for a constant ratio of neff/np over the wavelength range.

FIG. 5A shows waveguide index profile design for single angle, broadband coupling. A uniform M_(g)F₂ gap (coupling) 101 layer was used with a core SiON waveguide 102. Four additional cladding (photovoltaic) layers 103 were also provided. The relative thicknesses of the layers are shown in the index profile plot of FIG. 5B.

The optimum effective index of the waveguide of FIG. 5A is shown in FIG. 6A, along with a nominal single layer cladding of SiO2, and some design iterations of the cladding. The corresponding input beam width diagram for the ideal dispersion curve is shown in FIG. 6B. The nominal un-optimized single cladding and 5-layer multicladding beam width diagrams are shown in FIG. 6C and FIG. 6D, respectively. While this arrangement provides approximate beam spread to 1 degree, further optimization of the cladding layers is expected to reduce the beam spread to within 0.1 degrees.

8. Embedded Dichroic Filter

A multijunction photovoltaic structure that monolithically integrates the waveguide while physically and spectrally separating the junctions is shown in FIG. 7.

It is known that the lattice and current matching constraints of monolithic multi-junction solar cells limit their efficiency, while mechanically stacking the junctions or spatially separating the junctions allows more freedom of cell design and band gap optimization for improved efficiency. However, mechanically stacking these cells increases the complexity by introducing additional insulation layers and the handling and processing of thin junction layers. Very often the cells must go through substrate removal, carrier bonding, and thin wafer handling which are not trivial and affect the yield and cost of the devices. Also, creating separate junctions accessed by refractive optics and filters have alignment sensitivities that increase with the number of cells. Shown here is a way to utilize a special monolithically grown multi-junction cell in a spatially and spectrally separated waveguide that retains the simplicity of the monolithic multi-junction cell, while providing the benefits of spatial and spectral separation.

The geometry of a device such as shown in FIG. 7 includes a waveguide input coupling mechanism (prism 100), a waveguide core 102, a custom dual cell detector 110 and a dichroic filter 120. Each element of the dual cell detector 110 can be a single junction or a multi-junction cell. The flow of light is governed by evanescent coupling to each element of the dual cell and the dichroic filter above the first cell 110-1. The unique element of the design is the etched geometry of the dual cell and the embedded pseudo-omnidirectional multilayer dielectric mirror above the lower bandgap cell 110-2 (absorbing >660 nm). This multilayer, dielectric pseudo-omnidirectional mirror fits within the depth of the higher bandgap cell 110-1 (absorbing <660 nm), and is embedded by etching out an area outlined by the depth of the high bandgap cell and the evanescent coupling length of the higher wavelength band.

Unlike prior art that seeks omnidirectional reflection across a broad wavelength region, the design of this dichroic filter stack 120 is specific to the mode angles used within the waveguide commensurate with the solar spectrum. This skews the angles of interest to those exceeding the critical angles within the waveguide. The waveguide design is also unique in that it minimizes dispersion across the solar spectrum when used in conjunction with the particular prism (material dispersion).

The specific geometry used within this integrated device, and the integration of the waveguide 102, filter 120, and photovoltaic cells 110 make this device unique. It is the only known multi-junction solar cell device that incorporates light concentration, filtering and band gap optimized photovoltaic conversion in one device. The device is producible via standard wafer fabrication techniques such as metal oxide chemical vapor deposition (MOCVD) and reactive ion etching (RIE).

Details of the construction of the dichroic filter stack are shown in the illustration below the main element. Alternatively, layers of TiO2 and SiO2 are provided with the illustrated thickness and indexes to provide the illustrated reflected spectral response (Wo±Wo/3). Here the thickness of the short wavelength absorbing region may need to be extended via a buffer layer 250 to match the thickness of the filter if the filter is thicker than the short wavelength absorbing region.

9. Mirrors and Optional Dichroic Filters

FIG. 8A depicts broadband light that has been coupled to a multi-layer waveguide 102, 103 via a prism 100, as before. However in this arrangement, the light collected in the waveguide 102, 103 is coupled to cell(s) 105 via a set of longitudinally spaced, angularly etched TIR mirror(s) 130. Since the size of the spacing between mirrors 130, and hence the linear dimension of light at the prism aperture associated with this length, can be designed to be less than the solar coherence diameter, this device serves as a coherent solar array concentrator. It can use either standard energy gap conversion based cells 105 such as silicon or gallium-arsenide (GaAs) photovoltaics, or it can be used with metal-insulator-metal (MIM) tunnel junction rectifiers for direct current output as described below.

The conversion devices (detectors) 105 can be discrete devices, one for every mirror element 130 as per FIG. 8B, or they can be designed to be continuous monolithic structure and defined by their electric contacts 133 as shown in FIG. 8A.

An enhanced version of this device can further include intrawaveguide elements that enable the spectral separation of light into different photodetectors operating in different bands. FIG. 9, a more detailed view of each element, shows an instance of incoming light being separated into two (2) wavelength bands using an intrawaveguide dichroic multilayer filter 120. This implementation uses two detectors 161, 162 (InGaAg for the one band and GaAs for the other band), but this can be extended to multiple bands (for example, by disposing additional InGaAs and/or GaAs detectors beneath those shown).

FIG. 9 also shows an equation for the desired angle of the TIR mirror 130 as depending on the lowest index of refraction of the waveguide structure (approximately 1.5 here) resulting in a 45° preferred angle. The angle for the dichroic filter 120 may also be the same or different, depending on the arrangement of the detectors used and their position.

A simulation of the waveguide incorporating both the TIR mirrors 130 and a nominal low Q multilayer dielectric filter shown generally in FIG. 9 is shown in more detail in FIG. 10. The components may be fabricated as part of the plasma-enhanced chemical vapor deposition (PECVD) using standard CMOS manufacturing techniques of deposition and dry and wet etching.

HFSS simulations show that the light spectrum is selectively directed to the optimized detector via these intrawaveguide components.

The FIG. 10 simulation was of a two (2) wavelength band device with an interwaveguide dielectric filter that passes short wavelengths and reflects longer wavelengths (element #4). The longer wavelengths are directed to a smaller energy bandgap detector (or MIM array) while the shorter wavelengths pass through a single layer waveguide and are reflected to a higher bandgap detector via the TIR mirror etched ino the waveguide. The photos show the path of light taken by three (3) different wavelengths within the solar spectrum. The long wavelength reflection of the filter is clearly shown by 814 nm light being directed downward and entering the right most conversion cell, while midband light at 600 nm is split between the two (2) cells.

In measurements taken of one embodiment adapted for optical signal detection (indoors), five discrete laser wavelengths (405 nm, 632 nm, 808 nm, 532 nm and 975 nm) having a maximum input angle of 0.35° resulted in an angle measurement accuracy of +0.1° for a normalized beamwidth from 0.4 to 2.5°.

Note that a multi-layer waveguide may preferably be used as was described in FIGS. 5A and 5B with a core 102 and multiple “cladding” layers 103. In this instance, the detectors are located below the waveguide, rather than within it, as was the case for the FIGS. 3A and 3B embodiments. Thus, comparing the positions of the layers 102, 103 and detectors 105 in the various arrangements depicted in FIGS. 1, 3A, 3B, 5A, 5B, 8A, 8B, and 9, it should be understood that the drawings are not necessarily to scale for all aspects of all possible embodiments. The thickness of the various layers 102, 103 depicted for the waveguide and the detectors 105 may depend on the materials used and thus may be different than that suggested by looking at the drawings alone. For example, in one embodiment, the layers 103 may range from 100 nm to 1000 nm thick but the overall dimensions of the detectors 105 may be somewhat larger than any given layer 102, 103. Also, other materials may be used than suggested here for the layers 102, 103. In addition, the detectors 105 are not limited to band gap driven semiconductor structure, but can be any electron generating junction (e.g. tunnel junction). Furthermore, additional geometric and spectral separation can be achieved by lengthening the single layer waveguide 140 and optimizing the design of the dielectric filter 120. Thus it should be understood that the primary components of the collector design are the multi-layer wideband waveguide 102, 103, the adjacent detectors 105, and the optional features such as the dielectric filters 120, mirrors 130, and so forth.

10. MIM Rectennas

FIG. 11A is a cross sectional view of a MIM type rectifier that can be used as a detector (cell) 105 in certain embodiments. The MIM rectifier includes a silicon (Si) substrate 1101, and an silicon dioxide (SiO₂) buffer layer 1102. The MIM junction 1110 is disposed on the buffer layer 1102 is provided by a first metal layer (Metal 1) 1111 and second metal layer (Metal 2) 1112, a separated by a insulator 1113 such as a native oxide metal (Native Oxide Metal 1).

Here a thin layer (1-2 nm) of graphene 1114 is added to the metal-insulator-metal junction 1110, such as between the Native Oxide layer 1113 and Metal 2 layer 1112. The graphene layer 1114 may be custom-prepared, doped or undoped.

The addition of the graphene or doped-graphene layer improves the asymmetry of the current to voltage characteristics of the tunnel junction 1110, bringing the electrical response closer to that of an ideal diode.

The graphene layer 1114 may be replaced with other non-oxidizing semi-metals.

The resulting MIM diode 1100 finds particular use in antenna coupled waveguide dielectric or even simple separate high frequency rectifiers for IR energy collection.

In one embodiment, one or more MIM rectennas 170 fabricated with a graphene layer per MIM junction 1100 are placed vertically in series underneath the waveguide TIR mirror (as per FIG. 11B). Different layers 170-1, . . . , 170-n of such rectennas, tuned to a specific desired wavelength and bandwidth will be separated by some vertical distance. The spectral separation comes from the selectivity and sensitivity of each nantenna layer 170 to its corresponding wavelength. Existing antenna design principles allow the implementation of the tuned layers to be a straightforward procedure, allowing for any number of selective layers, depending on an efficiency/bandwidth trade.

It was shown in a full-wave FEM simulation (HFSS) that the stacking of different bands of antennas in a vertical arrangement causes little effect on the other band. In FIG. 12, two MIM “bowtie” antennas (antenna 1 and antenna 2 labeled “Ant1” and “Ant2” in the drawing) of two different bands (low and high) were stacked vertically. An incident wave entering from the top surface of the problem illuminates the system. Energy for which antenna 2 is tuned to are passed through antenna 1 and are collected on antenna 2 with little power loss or scattering. The traces in FIG. 12 clearly show the presence of a lower wavelength band nanttenna above a higher wavelength band nantenna causes very little shadowing or scattering. This feature allows for spectral separation of tuned nantennae in a vertical arrangement for any number of layers of tuned antenna with a bandwidth appropriate for the layer arrangement. The concept is analogous to a log-periodic antenna.

The MIM bowties could be arrayed periodically in x and y, and in z (separated by different layers or groups of layers corresponding to different frequency bands) as shown in FIG. 13A. Arrayed in this way, the bowties 170 can collect energy incident across a wide x-y plane, rather than having just a single nantenna at the center. For a given band, it will take more than a single layer of bowties 170 to collect 100% of the incident energy. Additional layers of the same band can be stacked before, after or interleaved with other band layers. Also in FIG. 13B, an HFSS power field cross section is shown, showing energy out of band for the first layer of bowties passing through undisturbed, and further being absorbed by the bowties in-band on lower layers.

FIG. 14 is a microphotograph of an integrated MIM bowtie antenna element.

FIG. 15 is a plot of expected receive efficiencies for bow tie elements of different sizes.

It can thus be understood that, in lieu of lateral spectral filtering along the waveguide as described above, vertical spectral filtering can be accomplished with the TIR mirror alone, and the filtering occurs naturally by the propagation of light through resonant arrayed MIM nantenna structures or even bandgap optimized multijunction solar cells.

The vertical array stack of FIG. 13A has three bands of rectenna arrays. The bands are arranged in a vertical stack with their spectral bandwidths shown in FIG. 15. In this way the MIM nantennas cover most of the solar spectrum from 400 nm to 1200 nm (250 THz to 750 THz). This is analogous to the long periodic RF antenna shown in FIG. 13A.

The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.

While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims. 

What is claimed is:
 1. An electromagnetic energy collector apparatus comprising: a planar waveguide having a top surface, a bottom surface, and a waveguide core, the waveguide core having a propagation axis parallel to the top surface and the bottom surface, the waveguide core further including multiple material layers having at least two different dielectric constants; a prism having two faces at an acute angle relative to each other with one of those faces disposed on the top surface of the planar waveguide and coextensive with the propagation axis of the planar waveguide, for providing electromagnetic energy to the planar waveguide; two or more mirrors formed within the waveguide core, each of the mirrors having a surface disposed at a critical angle to the propagation axis, and each mirror surface extending from at least a top surface to a bottom surface of the waveguide core, and each of the mirrors disposed in parallel with at least one another mirror; and at least one metal-insulator-metal (MIM) rectifier aligned with each mirror and disposed beneath the bottom surface of the planar waveguide, further comprising an insulator consisting of a native oxide and a graphene layer.
 2. The apparatus of claim 1 wherein a plurality of MIM rectifiers are disposed in a column beneath each mirror, each of the plurality of MIM rectifiers vertically aligned with at least one other MIM rectifier.
 3. The apparatus of claim 2 wherein an active wavelength range of a MIM rectifier disposed closer to a respective mirror is lower than an active wavelength range of a MIM rectifier disposed farther away from the same respective mirror.
 4. The apparatus of claim 3 wherein a plurality of MIM rectifiers are arranged in rows and columns beneath each mirror to provide an array of MIM rectifiers beneath each mirror.
 5. The apparatus of claim 4 wherein each array of MIM rectifiers is further arranged such that an active wavelength range of a group of MIM rectifiers in a row disposed closer to a respective mirror is lower than an active wavelength range of a group of MIM rectifiers disposed in a row farther away from the same respective mirror.
 6. The apparatus of claim 5 wherein the group of MIM rectifiers in the first row are disposed in a different substrate layer than the group of MIM rectifiers disposed in the row farther away from the same respective mirror.
 7. The apparatus of claim 1 where the graphene layer is doped.
 8. The apparatus of claim 1 wherein a dielectric constant of a material used to form each of the mirror surfaces differs from a dielectric constant of an area adjacent each respective one of the mirror surfaces to provide Total Internal Reflection (TIR) of energy reflected by the mirrors.
 9. The apparatus of claim 1 wherein the mirror surfaces are formed on a planar surface etched at an angle into the waveguide.
 10. The apparatus of claim 1 wherein the MIM rectifiers are bowtie elements.
 11. The apparatus of claim 1 further comprising: a coupling layer disposed between and coextensive with the prism and the waveguide.
 12. An electromagnetic energy collector apparatus comprising: a planar waveguide having a top surface, a bottom surface, and a waveguide core, the waveguide core having a propagation axis parallel to the top surface and bottom surface, the waveguide core further including multiple material layers having at least two different dielectric constants; a prism having two faces at an acute angle relative to each other with one of those faces disposed on the top surface of the waveguide and coextensive with the propagation axis of the planar waveguide, for providing electromagnetic energy to the planar waveguide; two or more mirrors formed within the waveguide core, each of the mirrors having a surface disposed at a critical angle to the propagation axis, and the mirror surface extending from at least a top surf ace to a bottom surface of the waveguide core, and each of the mirrors disposed in parallel with at least one another mirror; and at least one metal-insulator-metal (MIM) detector aligned with each mirror and disposed beneath the bottom surface of the planar waveguide.
 13. The apparatus of claim 12 wherein the mirror surfaces are formed on a planar surface etched at an angle into the waveguide.
 14. The apparatus of claim 13 wherein a dielectric constant of a material used to form each of the etched mirror surfaces is greater than a dielectric constant of air adjacent each of the etched mirror surfaces.
 15. The apparatus of claim 12 wherein a plurality of MIM detectors are disposed in a column beneath each mirror, each of the plurality of MIM detectors vertically aligned with at least one other MIM detector.
 16. The apparatus of claim 15 wherein an active wavelength range of a MIM detector disposed closer to a respective mirror is lower than an active wavelength range of a MIM detector disposed farther away from the same respective mirror.
 17. The apparatus of claim 16 wherein a plurality of MIM detectors are arranged in rows and columns beneath each mirror to provide an array of MIM detectors beneath each mirror.
 18. The apparatus of claim 17 wherein each array of MIM detectors is further arranged such that an active wavelength range of a group of MIM detectors in a row disposed closer to a respective mirror is lower than an active wavelength range of a group of MIM detectors disposed in a row farther away from the same respective mirror.
 19. The apparatus of claim 15 wherein at least one of the MIM detectors is active in a selected one of a 125, 187, and 250 nanometer (nm) range.
 20. The apparatus of claim 12 wherein a dielectric constant of a material used to form each of the mirror surfaces differs from a dielectric constant of an area adjacent each respective one of the mirror surfaces to provide Total Internal Reflection (TIR) of energy reflected by the mirrors.
 21. The apparatus of claim 12 wherein the MIM detectors are bowtie elements.
 22. The apparatus of claim 12 further comprising: a coupling layer disposed between and coextensive with the prism and the waveguide. 